1. Field of the Invention
This invention relates to the removal of wellhead assemblies and, more particularly, to the cutting of well casing below a wellhead to enable removal of the wellhead.
2. Description of the Related Art
When an oil or gas well is to be abandoned the usual procedure adopted is to plug the well with a suitable cement composition, test the integrity of the plug, and then remove the wellhead assembly. On land, the wellhead assembly can be removed by standard construction techniques and in general the casing immediately below the wellhead will be cut off several meters below ground level to allow reinstatement of the well site. This technique cannot satisfactorily be applied to subsea wells.
It is accordingly known, in the case of a subsea well, to plug the well bore with cement and then to detonate an explosive charge within the well casing slightly below the level of the wellhead proper in order to cut the casing at that point and free the wellhead assembly for removal. This technique is, however, unsatisfactory since the portions of the wellhead removed after explosive cutting are generally damaged and not suitable for re-use.
There is disclosed in EP-A-436706 a technique for cutting and removal of a wellhead by means of a rotating cutting tool mounted at the end of a drill string which is guided through the wellhead by means of a recovery tool. Because this solution requires the use of a drill string it is only applicable to wellheads which are located beneath an appropriate drilling rig. Although, in theory, this method could be applied to a wellhead which is not located beneath a drilling rig by bringing a drilling barge over the wellhead and running the necessary drill string from the drilling barge, such a technique would be prohibitively expensive given the high cost of operating a drilling barge.
The present invention is accordingly concerned with a wellhead removal technique which can be used during abandonment of a subsea oil or gas well and which does not require the use of explosive charges or of a drilling rig located above the wellhead.
According to one aspect of the present invention there is provided a method of cutting a well casing located beneath a subsea wellhead, the method comprising the steps of running a cutting tool through the wellhead into the casing; securing the position of a motor relative to the wellhead so as to transmit rotary reaction forces from the motor to the wellhead or a fixture secured relative to the wellhead; and operating the motor so as to rotate the cutting tool and thereby effect cutting of the casing.
Because, in the case of the present invention, the reaction force from the motor driving the cutter is reacted on to the wellhead, no rigid coupling from the wellhead to the surface is required. All power required for operation of the cutting tool (including power required by the motor) can be delivered to the wellhead by flexible connections extending from a support vessel located on the surface of the sea close to the wellhead. After the casing has been cut conventional lifting equipment may be used to remove the wellhead assembly.
The invention will be better understood from the following description of a wellhead abandoned operation, particular reference being had to the drawing which shows a casing cutter which may be used in an embodiment of the invention.
The present invention provides a method for recovering the wellhead from a subsea well in a way which does not require the use of explosives nor the presence of a drill string extending from the wellhead to a surface rig. The invention may be practised by use of a Multi-Functional Support Vessel (MSV) which can carry all the equipment and services required to effect abandonment of the well. In the alternative, abandonment may be completed using several vessels, for example one vessel responsible for initial cementing and testing of the well, one vessel for effecting the cutting of the casing, and a third vessel carrying appropriate lifting equipment for removal of the wellhead from the seabed.
In a particular embodiment of the present invention appropriate to abandonment of wellheads in the North Sea, for example in the North East Frig oil field, it is envisaged that all the services required to effect abandonment, save the final lifting of the wellhead assembly, will be completed from a single MSV. Such a vessel can be mobilized with all necessary equipment form a convenient port and can locate the required wellhead by a combination of navigation from Global Positioning System coordinates and sonar scanning. Typically, a submersible vehicle will be used for final wellhead identification and to carry out a survey of the wellhead site prior to commencement of the abandonment operation.
Once on location, a bell will be deployed enabling a diver to prepare the wellhead for the abandonment operation. Such preparation will typically include the removal of a corrosion cap by means of the main MSV crane.
The vessel will then be positioned to allow guidelines to run from the working moonpool to the guideposts of the wellhead.
A subsea wireline lubricator complete with tubing stinger will then be deployed to depth clamping the hydraulic control umbilicals to a pod line when approximately 6 m above the wellhead. The passive compensator will be opened and pressurised to operating tension, under appropriate monitoring. The subsea wireline lubricator will be fully functioned and pressure tested and cement hoses will be run from the MSV by means of a utility crane. Divers will connect hoses to the crossover valves and appropriate cementing operations will be completed. The actual nature of the cementing operation is not critical to the present invention. The object of the cementing operation is to cement the well bore and pressure test the resultant plug to ensure that the well is sealed when the wellhead and the few meters of casing immediately below the wellhead have been completely removed. Once cementing has been completed all cement equipment together with the guidelines can be removed to clear the wellhead. The abandonment operation on the particular wellhead cemented may then be performed. In the alternative, the vessel may complete cementing operations on several wellheads before returning to the first wellhead to continue with the abandonment procedure.
The next stage in the abandonment procedure comprises removal of any seal assembly blocking access to the well bore. The seal assembly may be removed by any conventional technique.
Next, a cutting assembly according to the present invention is deployed. The cutting assembly comprises a mechanical cutter for cutting at least the innermost casing of the well some few (2-3 typically) meters below the wellhead. Typically, such a tool would comprise radially expandable blades which, in use of the tool, are forced radially outwardly into engagement by the casing by means of hydraulic pressure. A typical embodiment of cutting apparatus, deployed on a wellhead, is shown in the drawing. The actual casing cutter 22 will, in practice, be located at the lower end of the tubing string 8. The exact design of the cutter is not critical to the present invention and any appropriate design of rotary cutter having expandable cutters for cutting well casing etc. may be used.